This invention relates to multilayer heat shrinkable film.
This invention has particular but not exclusive application to packaging for foods such as meat, poultry, and cheese and for illustrative purposes reference will be made to such application. However, it is to be understood that this invention could be used in other applications, such as packaging for curable putties and sealants, other foodstuffs such as tofu and the like.
Heat shrinkable films for perishables, such as meat, generally have a heat sealable inner layer and an outer abuse layer. The inner and outer layers are generally composed of the same resin, since resins of different melting points generally do not have the same shrinkage. This is to ensure that the shrink of the inner and outer layer is balanced. If the shrink of the inner and outer layers is not balanced, during the biorientation step, the secondary bubble is not stable and the film tends to curl inward or outward. In other words, if a high shrinkage resin in the inner layer is combined with a low shrink outer layer, or vice versa, the resultant film will curl or roll up and will not be able to be fabricated into bags.
In these types of films, the composition of the outer layer is chosen to be the same as the inner layer in order to achieve adequate seal strength and to prevent curl. However, an abuse outer layer having the same composition as the inner layer, leads to problems of burn-through and the need for irradiation. When heat sealing a film to make a bag, the seal area often becomes extruded during sealing. This elongation of the product results in thinning of the film and in an extreme situation severing of the thinned film. This is known as burn-through. One solution to this burn-through problem is to irradiate the film prior to manufacture of the bag.
Irradiation of a multilayer film causes the polymers in the film to cross-link. However, not all cross-linked thermoplastic films are easy to melt, which causes a reduction in heat sealability at conventional temperatures, pressures and times when using typical heat sealing equipment, thereby resulting in seals in bags made from the film. The equipment to irradiate the film is also highly expensive, costly to operate, increases the product cost, and usually requires an added step in the production operation.
International Publication No. WO00/32393 describes an attempt to overcome the problems of the above prior art films. There is disclosed a multilayer heat shrinkable film having an outer layer composed of a blend of thermoplastic polymers to provide a higher average melting point material than the inner layer, but having comparable shrink to prevent curling. The high melting point outer layer prevents burn-through. This allows a multilayer heat shrinkable film to be produced with excellent heat sealability while maintaining a high puncture resistant, high temperature outer layer to obtain a broad heat sealing range without the need to cross-link either the inner or outer layer. However, the film lacks the necessary clarity, gloss and haze requirements for a film for many products.
Accordingly, this invention in one aspect resides broadly in one aspect of the invention in a heat shrinkable multilayer film including:
a heat sealable inner layer of a thermoplastic polymer or a thermoplastic polymer blend;
an abuse layer of a thermoplastic polymer blend having a melting point at least about 20xc2x0 C. higher than said inner layer, said abuse layer blend being selected to have substantially balanced shrinkage relative to said inner layer, and
a thermoplastic, optically clear outer layer adjacent said abuse layer, wherein said outer layer has a lower Vicat softening point as determined by ASTM D-1525 than said abuse layer.
The Vicat softening point (VSP) in accordance with the ASTM D-1525 is the temperature at which a flat ended needle of 1 mm2 circular cross section will penetrate a thermoplastic specimen to a depth of 1 mm under a specified load and temperature. The test specimen is moulded or cut from a sheet with a specified minimum thickness and width. The specimen is placed on support bars and heated at a selected uniform rate of temperature rise. The needle is lowered so that it rests on the surface of the specimen. When the needle penetrates 1 mm the temperature is recorded, being the Vicat softening point. The data obtained from this test is useful in comparing the heat softening qualities of thermoplastic materials. Generally, thermoplastic products with a higher Vicat softening point tend to be harder to penetrate, whereas a lower Vicat softening point indicates a softer product.
Not being bound by theory, it is thought that films having an outer abuse layer with a higher melting point than the heat sealable inner layer, have less than desirable optical properties as the outer layer does not shrink as much as the inner layer. This may be so even if the inner and outer layers have compatible shrink such that the film does not curl. Since the outer layer is adhered to the inner layer, whether directly or via other layers of the film, it becomes wrinkled. This wrinkling causes the light to refract and makes the film appear hazy or white. When a softer material is added over the abuse layer it fills in the voids created by the wrinkles and allows the light to pass cleanly through the film. Since the light is not refracted the film appears clear.
The heat sealable inner layer may include very lower density polyethylene (VLDPE), linear low density polyethylene (LLDPE) ethylene vinyl acetate copolymer (EVA), ethylene xcex1-olefin plastomer copolymer (plastomer) or a blend on two or more thereof. Suitably, the LLDPE is produced using metallocene single site catalysts.
Suitable polymer blends for the heat sealable layer may include:
a blend of VLDPE, EVA, and plastomer, and
a blend of LLDPE, EVA, and plastomer.
The polymer blend may have a melting point in the range of about 94xc2x0 C. to 96xc2x0 C. Each of the aforementioned three component blends may include between about 20% to about 33% of VLDPE or LLDPE, between about 33% to about 60% of EVA, and between about 15% to about 30% of plastomer of the total blend. The EVA may contain between about 3% and about 18% vinyl acetate based on the weight of the copolymer.
The heat sealing inner layer may include other suitable polymers and copolymers such as polypropylene, ethylene-propylene copolymer or an ionomer. It is to be understood, however, that the thermoplastic polymers mentioned herein are not intended to be an exhaustive list, but merely exemplary.
Thermoplastic polymers that are suitable for use in the abuse layer are recognized by those skilled in the art. The thermoplastic polymers may be selected to enable a broad heat sealing range while preventing burn-through during impulse heat sealing. Suitably polymers that may be included in the abuse layer are polypropylene (PP), polybutylene (PB), plastomer and EVA. Examples of suitable polymer blends for the abuse layer may include:
a blend of PP and PB,
a blend of PP, PB and EVA, and
a blend of PP, PB and plastomer.
The abuse layer polymer blend may have a melting point in the range of about 115xc2x0 C. to 150xc2x0 C., more suitably in range of about 123xc2x0 C. to 136xc2x0 C. The VSP of the abuse layer may be in greater than 115xc2x0 C., more suitably about 119xc2x0 C. to 120xc2x0 C.
The abuse layer may include other thermoplastic materials, such as, for example, an ionomer, VLPDE, LLDPE or blends of these materials.
The optically clear outer layer may be a thermoplastic polymer or polymer blend. Suitable resins for use in the abuse layer may include PP, VLDPE, LLDPE, EVA, plastomer or suitable blends thereof. The LLDPE may be Ziegler-Natter catalyzed polymer. However, more suitably, the LLDPE is produced using metallocene single site catalysts.
Suitable polymers or polymer blends for the optically clear layer may include:
a blend of PP and plastomer;
a blend of VLDPE and plastomer;
a blend of LLDPE and plastomer;
a blend of VLDPE, EVA, and plastomer,
a blend of LLDPE, EVA, and plastomer, and
a blend of VLDPE, plastomer and PP.
As the VSP of PP can be relatively high, blends that include PP may include a substantially greater proportion of plastomer than PP. It is to be understood however that the thermoplastic polymers mentioned herein are not intended to be an exhaustive list, but merely exemplary. A person skilled in the art would be able to select a suitable polymer or polymer blend for the optically clear layer based on the composition of the abuse layer. In a preferred embodiment, the VSP of the optically clear layer may be less than about 111xc2x0 C. and, suitably less than 100xc2x0 C. In one embodiment, the VSP of the optically clear layer is less than about 80xc2x0 C. and, suitably in the range of about 70xc2x0 C. and about 80xc2x0 C.
One or more thermoplastic layers may be included between the abuse layer and the inner layer. For example, if the multilayer heat shrinkable film requires barrier properties, the inner and abuse thermoplastic layers may be are adhered to opposite sides of an oxygen barrier layer. The barrier layer may include PVDC, polyamide barrier material such as nylon and hydrolyzed ethylene vinyl acetate copolymer (EVOH). For example, the barrier layer may include vinylidene chloride-vinyl chloride copolymer, vinylidene chloride-methyl acrylate copolymer or blends thereof.
The thickness of the barrier layer may be between about 0.1 mils and about 0.5 mils (2.5-12.5 xcexc). Thinner barrier layers may not perform the intended functions and thicker layers do not appreciably improve performance.
The thickness of optically clear outer layer may be between about 0.2 mils and about 0.8 mils (5-20 xcexc). Thinner layers may result in poorer optical properties while thicker layers do not contribute to an improved film.
The thickness of the abuse layer may be between about 0.5 mils and about 1.0 mils (12.5-25 xcexc). Thinner layers may be less effective in performing the abuse resistance function, while thicker layers may reduce film stretchability.
The thickness of the heat sealable inner layer may be between about 0.4 mils to about 1.0 mils (10-25 xcexc) to perform the previously described functions.
A four-layer film according to the invention which includes an oxygen barrier layer may have a total thickness of from about 1.6 mils to about 3.0 mils (40-75 xcexc) and more suitably, from about 1.8 mils to about 3.0 mils (45-75 xcexc). Lower thickness may reduce the effectiveness of at least one of the four layers while higher thickness may reduce the film flexibility and may not appreciably improve its performance.
While the embodiment of the invention including a barrier layer is specifically described in terms of four layers, it should be understood that one or more additional thermoplastic layers may be included in the multilayer film. For example, one or more additional thermoplastic layers may be included between the barrier layer and the abuse layer, between the barrier layer and the inner layer, or in place of the barrier layer. This additional layer may include for example EVA, LLDPE, VLDPE, polypropylene, EVOH, polyurethane, acrylonitrile nylon, ionomer, or blends thereof. In addition, various conventional additives such as slip agents and pigments can be incorporated in the film in accordance with conventional practice.
Though a film in accordance with the invention does not require irradiation, it is to be understood that the film could be produced with one or more layers cross-linked by irradiation to further broaden and/or increase the heat sealing range.
The multilayer film of this invention can be produced by known techniques such as by co-extruding the multiple layers into a primary tube, followed by biaxially stretching the tube by known techniques to form a heat shrinkable film. Alternatively, the film may be a slot cast co-extruded multilayer film, which is subsequently biaxially stretched. In a still further alternative, the film may made by coating lamination, followed by biaxially stretching.
The resulting film may shrink from about 20% to about 60% in at least one of machine and transverse directions by measuring unrestrained shrink of the film at 90xc2x0 C. for about five seconds, or equivalent shrinkage thereof. In a preferred embodiment, the film shrinks at least 40% in at least one of machine and transverse directions.
The film of the present invention may be in the form of a flat sheet or a tubular article. Suitably, the film is formed into bags useful for packaging meat, poultry and cheese. The multilayer film is especially useful for those bags manufactured by hot bar, impulse sealing or a combination thereof.
In another aspect, this invention resides in a heat shrinkable multilayer film including:
a heat sealable inner layer of a thermoplastic polymer blend having an average melting point of between about 94xc2x0 C. to 96xc2x0 C.;
an abuse layer of a thermoplastic polymer blend having an average melting point of between about 120xc2x0 C. to 150xc2x0 C., said abuse layer blend being selected to have substantially balanced shrinkage relative to said inner layer, and
a thermoplastic, optically clear outer layer adjacent said abuse layer, wherein said outer layer has a lower Vicat softening point as determined by ASTM D-1525 than said abuse layer.
The film may include an oxygen barrier core layer between the inner layer and the abuse layer. For example, the barrier layer may be PVDC. The inner, abuse and outer layers may include any suitable thermoplastic polymer or polymer blend as described above. The optically clear outer layer may have a VSP of less than about 111xc2x0 C. In one embodiment, the optically clear outer layer has a VSP of less than about 80xc2x0 C.
In another aspect, this invention resides in a heat shrinkable multilayer film including:
a heat sealable inner layer of a polymer blend of VLDPE, EVA and plastomer;
an abuse layer of a polymer blend PP, PB and EVA or plastomer, said abuse layer blend being selected to have substantially balanced shrinkage relative to said inner layer, and
an optically clear outer layer adjacent said abuse layer of a polymer blend of EVA, VLDPE and PP.
The film may include an oxygen barrier core layer between the inner layer and the abuse layer. For example, the barrier layer may be PVDC. The inner, abuse and outer layers may include any suitable thermoplastic polymer or polymer blend as described above. The optically clear outer layer may have a VSP of less than about 111xc2x0 C. In one embodiment, the optically clear outer layer has a VSP of less than about 80xc2x0 C.
In order that this invention may be more readily understood and put into practical effect, reference will now be made to the following examples which illustrate preferred embodiments of the invention.
As used herein, the term xe2x80x9cpolymerxe2x80x9d refers to the product of a polymerization reaction, and is inclusive of homopolymers, copolymers, terpolymers, etc.
As used herein, the term xe2x80x9chomopolymerxe2x80x9d is used with reference to a polymer resulting from the polymerization of a single monomer, i.e., a polymer consisting essentially of a single type of repeating unit.
As used herein, the term xe2x80x9ccopolymerxe2x80x9d refers to polymers formed by the polymerization reaction of at least two different monomers. For example, the term xe2x80x9ccopolymerxe2x80x9d includes the copolymerisation reaction product of ethylene and an xcex1-olefin, such as 1-hexene. However, the term xe2x80x9ccopolymerxe2x80x9d is also inclusive of, for example, the copolymerisation of a mixture of ethylene, propylene, 1-hexene, and 1-octene.
As used herein, the phrase xe2x80x9cethylene xcex1-olefin copolymerxe2x80x9d refer to such heterogeneous materials as linear low density polyethylene (LLDPE), and very low and ultra low density polyethylene (VLDPE and ULDPE); and homogeneous polymers such as metallocene catalyzed polymers such as EXACT(trademark) materials supplied by Exxon, and TAFMER(trademark) materials supplied by Mitsui Petrochemical Corporation. These materials generally include copolymers of ethylene with one or more comonomers selected from C3 to C10 xcex1-olefins such as propene, butene-1, hexene-1, octene-1, etc in which the molecules of the copolymers comprise long chains with relatively few side chain branches or cross-linked structures. Other ethylene xcex1-olefin copolymers, are available from the Dow Chemical Company, known as ENGAGE(trademark), AFFFINITY(trademark) and ATTANE(trademark) resins, are also included as another type of ethylene xcex1-olefin copolymer useful in the present invention.
As used herein, the term xe2x80x9cethylene vinyl acetate copolymerxe2x80x9d or xe2x80x9cEVAxe2x80x9d is used to refer to a copolymer formed from ethylene and vinyl acetate monomers wherein the ethylene derived units in the copolymers are present in major amounts, preferably between about 60% and 98% by weight and the vinyl acetate derived units in the copolymer are present in minor amounts, preferably between about 2% and 40% by weight.
As used herein, a xe2x80x9cplastomerxe2x80x9d is a low molecular weight ethylene xcex1-olefin copolymer which has properties generally intermediate to those of thermoplastic materials and elastomeric materials. Typically, plastomers have an ethylene crystallinity between LLDPE and VLDPE and ethylene xcex1-olefin elastomers. The plastomers used in this invention comprise copolymers of ethylene and at least one C3-C10 xcex1-olefin. Illustrative non-limiting examples of suitable copolymers are those such as ethylene butene-1, ethylene hexene-1, ethylene octene-1, and ethylene propylene copolymers.
As used herein, the term xe2x80x9cpolypropylenexe2x80x9d refers to any polymer comprising propylene polymerization units, regardless of whether the polymer is a homopolymer or a copolymer, and further includes blends of such homopolymers and copolymers.
As used herein, the term xe2x80x9cpolybutylenexe2x80x9d refers to any polymer comprising butene-1 polymerization units, regardless of whether the polymer is a homopolymer or a copolymer, and further includes blends of such homopolymers and copolymers. These materials generally include copolymers of butene-1 with or without xcex1-olefins monomers, such as butene-1-ethylene, butene-1-propylene, and butene-1-xcex1-olefins having from 5 to 8 carbon atoms.
As used herein, the term xe2x80x9cpolyvinylidene chloridexe2x80x9d or xe2x80x9cPVDCxe2x80x9d refers to a vinylidene chloride copolymer wherein a major amount of the copolymer comprises vinylidene chloride and a minor amount of the copolymer comprises one or more unsaturated monomers copolymerizable therewith, typically vinyl chloride, and alkyl acrylates (e.g. methyl acrylate) or to a blend thereof in different proportions. Generally, PVDC will contain plasticisers and/or stabilizers as known in the art.
As used herein, the phrase xe2x80x9couter layerxe2x80x9d refers to the outermost layer of the film which in the end package will be in contact with the environment.
As used herein, the phrase xe2x80x9cheat sealablexe2x80x9d means that the inner layer is capable of fusion bonding by conventional indirect heating means which generate sufficient heat on at least one film contact surface for conduction to the contiguous film contact surface and formation of a bond interface between without loss of the film integrity. The bond interface should be sufficiently thermally stable to prevent gas or liquid leakage when exposed to above or below ambient temperatures during processing of food within the tube when sealed at both ends, i.e., in bag form. Further, the bond interface between contiguous inner layers must have sufficient physical strength to withstand the tension resulting from stretching or shrinking due to the food body sealed within the tube.
As used herein, the phrase xe2x80x9cabuse layerxe2x80x9d means a film layer which resists abrasion, puncture, and other potential causes of reduction of package integrity relative to the other layers of the film.
As used herein the term xe2x80x9cbarrier layerxe2x80x9d means oxygen barrier layer unless otherwise specified. The barrier layer may serve to inhibit transmission of oxygen and water vapor. The functional requirement of the barrier layer, together with the other layers, may provide an oxygen transmission rate through the entire multilayer cook-in film below about 20 cc/1 m2/24 hrs/atm. This may avoid spoilage of certain food products, for example meat enclosed in the multilayer film package due to oxygen passage from the environment through the film wall.
As used herein, the term xe2x80x9csubstantially balanced shrinkagexe2x80x9d means that the inner and abuse layers are compatible in shrink such that the composite structure of the film does not curl either inward or outward. It is to be understood that the shrinkage of each separate inner and abuse removed from the composite structure of the film is not to be considered when determining when a film has substantially balanced shrinkage.
As used herein, the term xe2x80x9cmelting pointxe2x80x9d means average melting point when referring to a polymer blend. It is calculated by averaging the melting points of the thermoplastic polymers in the blend taking into account the proportion of each polymer in the blend.
As used herein, the term xe2x80x9cheat-shrinkablexe2x80x9d refers to a film which shrinks by at least 15% of its original dimensions, in at least one of the machine and transverse directions, when heated to 90xc2x0 C. for at least four seconds.
In a preferred embodiment, a heat shrinkable multilayer film was produced having:
a) a heat sealable inner layer of a blend of VLDPE or LLDPE and EVA and plastomer having a melting point of about 96xc2x0 C.;
b) a barrier core layer of PVDC;
c) an abuse layer of a blend of PB, PP, and a plastomer or EVA having a melting point of between about 123xc2x0 C. to about 136xc2x0 C.;
d) an optically clear outer layer of a thermoplastic polymer blend adjacent said abuse layer and selected to have VSP less than 111xc2x0 C.
In various embodiments of the invention, the outer layer was selected from the following:
a blend of EVA, VLDPE and a plastomer having a melting point of about 96xc2x0 C.;
a blend of EVA, LLDPE and a plastomer having a melting point of about 96xc2x0 C.;
a blend of VLDPE and a plastomer having a melting point of about 98xc2x0 C.;
a blend of LLDPE and a plastomer having a melting point of about 98xc2x0 C.; and
a blend of a PP and a plastomer having a melting point of about 83xc2x0 C.
a blend of VLDPE, plastomer and PP having a melting point of about 101.7xc2x0 C.
A preferred blend of polymers suitable for the optically clear outer layer, which is not listed in Table C, is composed of:
a) An ethylene xcex1-olefin plastomer copolymer having a melting point of 74xc2x0 C., a density of 0.885 gm/cm3, a melt index of 1.0 g/10 min, and VSP of 57xc2x0 C. The plastomer comprises about 45 wt % of the total blend of the outer layer.
b) A VLDPE comprising about 44 wt %, of the total blend of the outer layer. It is an ethylene-octene copolymer having a melting point of 122xc2x0 C., a density of 0.912 gm/cm3, a melt index of 1.0 g/10 min, and a VSP of 92xc2x0 C.
c) A polypropylene having a melting point of 135xc2x0 C., a density of 0.89 gm/cm3 and a melt index of 5.5 g/10 min comprising about 10 wt % of the total blend of the outer layer.
d) Processing aid comprising about 1 wt % of the total blend of the outer layer.
The heat sealable inner layer includes at least 33% to about 60% of EVA, at least 20% to about 33% of VLDPE or LLDPE, and at least 15% to about 30% of plastomer based on the total blend of the inner layer. The EVA may contain between about 3% and about 18% vinyl acetate based on the weight of the copolymer.
The thickness of optically clear outer layer is between 0.2 mils and 0.8 mils. The thickness of the abuse layer is between about 0.5 mils and 1.0 mils. The thickness of the barrier layer is between 0.1 mils to 0.5 mils. The thickness of the heat sealable inner layer is between about 0.4 mils to about 1.0 mils. The four-layer film has a total thickness of from about 1.6 mils to about 3.0 mils, more suitably from about 1.8 mils to about 3.0 mils.
In general, various conventional additives such as slip agents and pigments were incorporated in the films in accordance with conventional practice.
The multilayer film of this invention can be produced by known techniques such as by co-extruding the multiple layers into a primary tube, followed by biaxially stretching the tube by known techniques to form a heat shrinkable film. The xe2x80x9cdouble bubblexe2x80x9d technique disclosed in Pahlke U.S. Pat. No. 3,456,044 can be used to produce the film of this invention. Alternatively, the film may be a slot cast co-extruded multilayer film, which is subsequently biaxially stretched. In a still further alternative, coating lamination may be used to for the multilayer film. For example, a first inner tubular layer may be extruded and thereafter the barrier, abuse, and outer tubular layers are sequentially coated onto the inner to form the composite primary tube, which may be biaxially stretched. Coating lamination procedures are described in Brax et al. U.S. Pat. No. 3,741,253.
If the film of this invention is produced in a tubular stock, bags can be produced by hot bar sealing one end of a length of the tubular film or at any number of longitudinally spaced positions across the tube width, and then cutting the tube or splitting one edge to form the bag mouth. If the film of this invention is made in the form of flat sheets, bags can be formed by hot bar sealing three edges of two superimposed sheets of film. When carrying out the hot bar sealing operations, the surfaces that are heat sealed to each other to form seams are the inner heat sealable layers of the multilayer films of the invention. The inner layer forms the inside portion of the bag while the outer layer forms the outside portion of the bag. The mouth of the bag may be sealed by impulse heat sealing after filling. Once food is inserted into the bag, the package is evacuated and the bag mouth sealed, generally by impulse heat sealing techniques.